Insufficient soft tissue tension:
The main soft tissue that keeps the hip securely in place is the abductor muscle.
This is a large muscle that is attached to the pelvic bone at the one end and
to the top aspect of the outer prominence of the hip or femur bone on the other.
It enables an individual to move the leg sideways away from the body. The distance
between these two points of attachment keeps the muscle stretched and is responsible
for the resting tension, not unlike an elastic band which has some tension in
it if stretched.
During the hip replacement operation, the natural hip joint is removed. This allows the two points of attachment of the abductor muscle to come together with loss of tension. When the surgeon implants the hip, the two points again move out to their "normal" location allowing the tension to be recreated and this helps in keeping the hip joint in position. There is the opportunity to carry out a trial hip replacement using purpose designed components before the final implants are seated and so the surgeon is able to fine tune the artificial hip to the requirements of the individual patient and check for adequate stability and tension.
If for any reason the necessary tension has not been recreated, then the hip is at risk of dislocation. This may arise for example, if the two points of attachement of the muscle are not as far apart as they were prior to the surgery, if there is preexisting weakness in the muscle such as when there have been multiple previous operations or if there is a developmental problem, there is a fracture or break in the femur bone at the point of attachment of the abductor muscle or if there is inadvertent damage to the muscle at the time of surgery.
It may be possible in some cases to recreate the soft tissue tension by putting in a longer piece (ball) between the femoral component and the socket component of the hip replacement or using a purposed designed bearing on the socket side. In other cases, this may not be sufficient and the femoral component may have to be redone. If there is a fracture at the point of attachment of the abductor muscle then it may be possible to reattach or repair that and recreate stability in the hip. There are instances when none of these options work and a constrained socket may have to be used.
Back to surgery for dislocating hips
To return to the home page, click here.